| Abstract: | The protoplasmic-streaming theory of phloem transport has beenextended to include a transcellular streaming concept. Phloemexudate, concentration gradients, and the respiratory energyrequirement are interpreted in terms of this concept. Physiological and cytological experiments are considered, andit appears that none of the unequivocal results contradict theproposed model. Reconciliation of apparently conflicting evidenceis possible because transcellular streaming is primarily a fluidflow phenomenon and is directly dependent on respiration forits motive force energy. As in other theories, it is difficult to understand how themeasured rates of mass transfer occur through the phloem. Thepresent model may be able to meet quantitative demands becauseof increased efficiency due to the direct linear pathways providedby transcellular strands, and the increased cross-section ofsieve-tube cytoplasm. More facts are required before the masstransfer problem can be resolved. |
